Method of precipitation hardening of copper-aluminum alloys

ABSTRACT

A method of precipitation hardening of Cu-Al alloys, which comprises the steps of subjecting the alloys to the action of one or more electromagnetic alternating fields whose frequency is of a defined value and simultaneously applying heat to the alloys in a single temperature zone which is lower than the eutectoid temperature of the alloys, and finally quenching the alloys to ambient temperature by an aqueous coolant. The entire process takes place at a much faster rate, and the structure of the resulting alloy has a finer grain and greater resistance to corrosion than that obtained by conventional precipitation hardening.

United States Patent 119 [111 3,801,382 Ettenreich 1451 Apr. 2, 19 74- [5 METHOD OF PRECIPITATION [56] References Cited HARDENING 0F COPPER-ALUMINUM UNITED STATES PATENTS ALLOYS 2,887,422 5/1959 Stone et a1 148/159 Ludwig Ettenreich, Wien, Austria Elin-Union A.G. fur elektrische lndustrie, Wien, Austria Filed: Dec. 6, 1971 Appl. No.: 205,055

Related US. Application Data Inventor:

Assignee:

Continuation-impart of Ser. No. 800,995,1 ebv 20, 1

1969, abandoned.

Foreign Application Priority Data Feb. 27, 1968 Austria 1854/68 US. Cl. 148/159, 148/32.5 Int. Cl. C22f 1/04 Field of Search 148/159, 32.5

Primary Examiner-Richard 0. Dean [57] ABSTRACT A method of precipitation hardening of Cu-Al alloys, which comprises the steps of subjecting the alloys to the action of one or more electromagnetic alternating fields whose frequency is of a defined value and simultaneously applying heat to the alloys in a single temperature zone which is lower than the eutectoid temperature of the alloys, and finally quenching the alloys to ambient temperature by an aqueous coolant. The entire process takes place at a much faster rate, and the structure of the resulting alloy has a finer grain and greater resistance to corrosion than that obtained by conventional precipitation hardening.

11 Claims, 3 Drawing Figures %MENTED R 2 i974 SHEU 2 BF 3 INVENTOR ludu i Eff e2 rea'cf m N nAPR 21914 (801; 382

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(g wmmadwa INVENTOR A adv Ezterz reici METHOD OF PRECIPITATION HARDENING OF COPPER-ALUMINUM ALLOYS This is a continuation-in-part of application Ser. No. 800,995,'filed Feb. 20, 1969, by the same inventor, titled Single-Stage Alloy Precipitation Hardening, now abandoned.

The present invention relates to precipitation hardening of copper-aluminum alloys in a single heating zone while subjecting the alloys to the action of one or more electromagnetic alternating fields.

It is known that a large number of alloys containing two or more components can be precipitation hardened. By this term a metallurgical process is understood which serves to increase the hardness and strength of the alloys.

The fundamental prerequisite for precipitation'hard? ening is that the main alloy element of a material should have the property of receiving the other alloy constituents, necessary for precipitation hardening, in

a state of solid solution. The dissolving power itself is dependent on temperature.

According to the present state of the art, precipitation hardening is effected in two or three successive stages, as follows:

1. Solution heat treatment (homogenization heat treatment) of the alloys, including quenching to ambient temperature By means of this treatment, precipitated crystals are brought to the form of a solid solution in the crystals of the main alloy constituent. The state of the'solution which exists at the temperature of the solution is at first maintained by quenching, even at room temperature. At this temperature, however, a supersaturated solid solution will exist which is no longer in thermodynamic equilibrium andtherefore tends to change into a saturated solid solution, with the formation of precipitates. The solution heat treatment followed by quenching will produce an increase of hardness and strength.

2. Age hardening at room temperature of the alloys which have been. subjected to solution heat treatment and quenching This treatment gives rise to segregation processes in the supersaturated solid solution, which lead to further spontaneous hardening.

3. Hot age hardening (annealing or tempering) of the alloys immediately after the quenching or age hardening at room temperature Tempering up to determined temperatures results in precipitations from thesolid solution in a highly disperse form. As long as the precipitates are in this form, further increase in hardness and strength takes place. If, however, a determined temperature, the so-called critical dispersion degree, is exceeded, the precipitates are no longer in highly disperse form but in the form of coarse grains so that a reduction in strength occurs.

Cold drawing, which is sometimes carried out after cold and/or hot age hardening and which leads to a fur- I endless strips in two, sometimes three different temperature zones. After a heating-up period, the alloys are maintained for a time of l A to 3 minutes in the same temperature zone within the solution heat-treating range, whereafter quenching is performed to a temperature below the solution heat-treating range.

As compared thereto, the process according to the invention has the advantage that it can be carried out regardless of the geometric forms of the alloys to be hardened, and that there is no retention period required since the heating-up step alone results in the metallurgical changes occurring in the alloys, bringing about the desired improvements in properties.

In contrast to the present state of the art, the invention consists of a method of precipitation hardening copper-aluminum alloys, comprising the steps of subjecting the alloys to at least one electromagnetic alternatingfield the frequency of which is derived from the formula f= 503 M H- p. expressed in c/s,

wherein d depth of penetration in millimeters, in proportion to the dimensions of the alloys,

u, relative permeability, and

H specific conductivity (Siemens) ohm cm, and simultaneously applying heat to the alloys in a single temperature zone which is lower than the eutectoid temperature of the alloys, preferably just below the eutectoid temperature, and finally quenching the alloys to ambient temperature by an aqueous coolant, preferably by water.

All parameters of the inventive process, as will be described later in more detail, such as the frequency itself, rethe rate of change in magnet polarity, the relative movement between the magnetic field and the alloy introduced into the field, the frequency selected for the heat applying step (in proportion to the alloy dimensions), can be derived readily from the basic formula according to the invention.

By the inventive treatment the hardness and strength of the alloys are increased in a manner which hitherto could be achieved only by solution heat treatment, including quenching, together with subsequent cold and hot age hardening.

The electromagnetic alternating fields applied according to the invention have the effect that the additional hardness and strength increase is obtained already during the solution heat treatment (including quenching). In other words, the metallurgical processes which hitherto took place only during cold or hot age hardening occur already at the high temperature of the novel solution heat treatment.

Moreover alloys treated according to the invention show no further increase of hardness or strength after subsequent cold or hot age hardening. These steps are therefore'made superfluous by the use of the present invention.

The structure of an alloy precipitation hardened according to the invention has a finer grain and greater resistance to corrosion than if it had been treated by conventional precipitation hardening.

With the application of the inventive method, the entire procedure is performed more rapidly than with conventional precipitation hardening.

Other objects and many of the attendant advantages of the invention will be readily appreciated as the same becomes better understood by reference to the following detailed description, when considered with the accompanying drawings, wherein FIG. 1 is a phase diagram showing the composition of an exemplary copper-aluminum alloy on which the inventive process can be carried out;

FIG. 2 illustrates one type of an inductive heating and observation apparatus built for laboratory use; and

FIG. 3 is a diagram showing the solubility of Cu in Al.

An example of an industrially important copperaluminum alloy will be used for describing the inventive process, designated in accordance with American Standards as 201 1-T3. Composition of the alloy is, in addition to aluminum: 5.4 to 5.9 percent Cu, 0.3 percent Fe, 0.15 percent Si, 0.05 percent Mg, 0.2 percent Zn, 0.3 to 0.6 percent Pb, 0.3 to 0.6 percent Ni, all percentages being given by weight.

Referring to FIG. 1, it can be seen that two types of crystals may occur, namely in region A, an a solid solution consisting mostly of aluminum and copper; and in region B, the a solid solution and a mixture of a with a second type of crystal B, an intermetallic compound Al Cu, copper aluminide.

The eutectoid temperature is at 548 C and at that temperature the aluminum is capable of dissolving about 5.7 percent Cu by weight.

The microhardness (load of a test diamond 100 p) of this alloy amounts to 77 to 81 kp per sq.mm, after solution heat treatment in a salt bath, cold and hot age hardening.

When a cold alloy, which is to be subjected to solution heat treatment, is introduced into a salt bath which is at the temperature of dissolution, the temperature will decrease, often to a substantial extent, depending on the amount of alloy being introduced and on the size of the salt-bath furnace; the time then required to reheat to dissolution temperature may be considerable, so that the output is reduced.

In a furnace containing 5,000 kg of salt bath, which consists of NaNO;,, KNO and K Cr O,, the temperature drops by 20 C on introduction of about 100 kg of the cold alloy, and the heating time required in that case to restore the dissolution temperature will amount to about 30 minutes.

With a dissolution temperature of 520 C, the retention time in the salt bath for the exemplary alloy will be one minute per each millimeter thickness of the material.

The alloy in question was subjected to the inventive process by exposing it to one (alternatively more) electromagnetic alternating fields(s) the frequency of which can be readily derived from the formula f(in c/s) 503 /11 H p wherein d depth of penetration in millimeters, in proportion to the dimensions of the alloy,

[L relative permeability of the alloy, and v H specific conductivity (Siemens) ohm cm, while heat was applied to the alloy in a single temperature zone lower than the eutectoid temperature thereof, and finally the alloy was quenched to ambient temperature by water.

As an example of the average value of the depth of penetration d forming part of the formula, the following practical values can be given: for a test rod of one meter length and 16 millimeters diameter, a frequency f of 1,500 c/s was applied. The relative permeability of the material was taken as u,- 1 since aluminum and aluminum alloys are not magnetic materials. The temperature coefficient of the resistance at 20C was a 10" per C. The specific conductivity H 41.55 (Siemensl per min specific resistance p= l/H=0.0 24l mm per m; average specific resistance between 0 and 500C, in due consideration of the temperature coefficient value a: pm 0.27 mm per m 0.

This was arrived at in the following manner: at 500C the average resistance increase is about 0.5 Q (500 10 .Q). The average specific resistance 7 which enters into the computation thus amounts to 0.024] 0.25 0.2741 0.27 mm per m 0.

= 5.03 V 27715 5.03 1.8 6.745, that is a depth of pene'tiatrsa'arwzs millimeters. Acaararn tothe S so -1111 wherein S (Alcm current density at a distance x from the surface (A amperes), I

S"(A/cm current density at the material surface,

and

x (mm) distance from the surface at which the current density should be determined; also d (mm) depth of penetration according to the calculations, and as mentioned earlier; and

e the exponential function.

Heat beyond the depth of penetration, in the direction toward the core of the material, is applied by way of conduction.

In the exemplary test described before the average depth of penetration value was exlained, the alloy was completely precipitation hardened, solely by the solution heat treatment and quenching. Heating to the dissolution temperature was effected by induction. Other, alternative procedural steps will be described somewhat later.

In order to permit continuous microscopic observation of the structure changes occurring during heating and quenching, an inductive heating apparatus for laboratory examinations was constructed so that it constitutes, at the same time, an inductively heated apparatus for microscopic observations. It is consequently possible for heating processes occurring at high speed to be continuously observed in the microscope and for all important parameters to be determined.

From FIG. 2 the laboratory-type construction of the inductive heating and observation apparatus can be seen. In the drawing, numeral 1 designates the test piece to be examined, which is in the form of a tube open at the top and closed at the other end, and the latter facing the objective of the microscope, schematically shown at 2. The thickness of the cylinder wall of the test piece is 1 mm, the thickness of the bottom surface to be observed is 2 mm, the diameter of the tube is 9 mm and the height of the entire test piece is 35 mm. Numeral 3 designates a protective glass between test piece 1 and objective 2. Test piece 1 is surrounded by a ceramic tube 4. An induction coil 5 is disposed around the latter. A tube 6, through which a quenching medium can be injected into test piece 1, projects into the latter. Arrow 7 indicates that a protective gas may pass between test piece 1 and ceramic tube 4. A thermocouple 8 serves to measure the temperature of the bottom surface of test piece 1.

The frequency of the induction current applied to coil 5 during the test, calculated on the basis of the above-explained formula, was about 7 kcls (kHz). The induction current was supplied by an RC generator and an amplifier.

For the purpose of inductive heating, the test pieces such as 1 were introduced into induction coil 5. Temperature measurement was effected by means of thermocouple 8 and a mirror galvanometer. The test pieces were heat treated in an atmosphere of a protective gas in order to prevent oxidation of the surface to be observed. After reaching the dissolution temperature the test pieces were quenched with water.

Before precipitation hardening as described, the test pieces were ground and polished on the observed surfaces. l-lalf of the test pieces were etched with NaOH before heating and the other half were left unetched for the purpose of thermal etching.

It was found that the thermal etching then actually occurring during the solution heat treatment gives the best elucidation regarding the metallurgical processes which occur.

More than 100 test pieces were subjected in the laboratory to precipitation hardening, with continuous observation through the microscope in all cases. The examinations were carried out in such a manner that one parameter at a time was varied while all other parameters were kept constant. These parameters are: 1. heating-up time; 2. dissolution temperature; and 3. quenchingtime.

The results of these examinations are as follows: After informative experiments to determine the approximate minimum or maximum heating-up times, heating up to the dissolution temperature was carried out in a time range from 5 to 60 seconds with a material thickness of 2 mm. The minimum time of 5 seconds could not be reduced because the available inductive heating equipment had already reached its limit capacity with this heating-up time. However, on the basis of the results of the tests, it can be stated that a further shortening of the heating-up time is possible but this must not be carried so far that the dissolution temperature, which is necessarily raised thereby, exceeds the solidus temperature.

From FIG. 3, one can see the percentage dissolving power of aluminum for copper in dependence upon temperature. There is a steady rise in dissolving power with rising temperature up to the eutectoid temperature of 5 48 C. in solution heat treatment, the eutectoid temperature must not be reached or exceeded since otherwise melting will commence. The maximum precipitation hardening effect is accordingly to be expected just below the eutectoid temperature.

The optimum dissolution temperature at the recited composition of the exemplary alloy was found to be about 545C.

The microhardness values after this precpitation hardening gave the same, and even slightly higher values, compared with conventional precipitation hardening. The micro-hardness (loading of test diamond p) was 81 to 83 kp per sq.mm.

The structure of the alloy, precipitation hardened in this manner, is finer than the structure after conventional precipitation hardening. This effect is explained by the rapidity with which the entire precipitation process takes place. Microsections showing the structure after conventional precipitation hardening still reveal grain limits.

If the hardening is carried out in accordance with the invention, no grain limits can be seen in the microsections. This shows that alloys precipitation hardened in this manner have greater resistance to corrosion than alloys treated in the usual manner.

Before precipitation hardening, oriented glide lines could clearly be seen in the fractures of the alloys. After inductive precipitation hardening, no glide lines could be seen, and the structure was of a homogeneous fine-grained nature. The quenching speed achieved with water was entirely adequate.

After the principle forming the basis of the present invention has been confirmed by laboratory tests such as described above, and the parameters have been determined which are of decisive importance to the process, the practicability of the method on an industrial scale was tested in a pilot plant.

Bar material was precipitation hardened in a continuous pass-through process in which the bars ran through an induction coil. The frequencies of the induction currents were from 500 to 2,000 c/s, depending on the diameters of the bars, and the necessary power for the precipitation hardening was about 300 kW per hour.

The dimensions of the bars, the throughput and the specific power can be seen from the following table.

With the same power of 300 kW, various throughputs were achieved in dependence upon the diameter of the bars. This is due to the fact that electromagnetic coupling is better with larger diameters.

For solution heat treatment alone without hot age hardening, with a salt bath containing 5,000 kg, a specific power of 0.52 kW/kg/h is required while the throughput per hour amounts to only about 100 kg.

The same results as in the laboratory were also obtained in the large-scale tests, i.e., maximum precipitation hardening in a single operational stage.

In the case of hitherto customary solution heat treatment, bar material must be cut into pieces after production and before being introduced into the salt bath. This operation is also eliminated when the method according to the present invention is applied to continuous heating processes.

As a further explanation of the invention, it should be stressed that the electromagnetic alternating fields used according to the inventive process have the effect that the additional increase of hardness and strength, which otherwise occurs only during cold and hot age hardening, are obtained during the solution heat treatment proper. The metallurgical processes which, according to the present state of the art, take place only during cold or hot age hardening, accordingly occur already at the high temperature of the novel solution heat treatment according to the present invention.

Moreover, an alloy which is precipitation hardened by the method of the present invention actually shows no further increase of hardness or strength, either after subsequent cold age hardening or after hot age hardening. These steps are therefore eliminated by the use of the inventive method.

The structure of an alloy precipitation hardened according to the present invention has a finer grain and greater resistance to corrosion than after conventional precipitation hardening.

When the method is applied, the entire precipitation hardening process takes place more rapidly than in the case of conventional hardening.

The advantages of the process are, first of all, considerable saving in time as compared to conventional hardening processes; moreover, the extremely finegrained structure of the alloys and the even distribution of the components, which lead to particular technological advantages; finally the dispensability of a subse quent cold or hot age hardening.

By the use of electromagnetic fields of the desired frequency, oscillations of the atomic lattice are affected, which contributes to the surprising effect of the invention.

The three important parameters described earlier are specifically defined, such as heating-up times in the order of seconds, dissolution temperature, being kept just below the eutectoid temperature, which has a different value for each alloy, and quenching time, again in the order of seconds.

Quenching is efi'ected generally by water of room temperature, i.e., about 20C. The quenching time, as mentioned, is in the order of seconds.

it can be summarized that the electromagnetic alternating fields are produced, according to the invention, in the alloys to be subjected to precipitation hardening in one of the following manners:

1. By electromagnetic induction. Eddy currents are produced in the alloys by electromagnetic induction. Like all alternating currents, these eddy currents are always accompanied by electromagnetic alternating fields.

2. By conductive supply of alternating currents. The alloys are conductively connected by appropriate terminals to a source of alternating current. Exactly as in the case of electromagnetic induction, alternating currents passing through the alloys produce electromag netic alternating fields therein.

In addition to electromagnetic induction or the passage of an alternating current through the alloys, there are further possible ways of producing electromagnetic alternating fields, such as:

a/ By converting ferromagnetic material into a magnet of continuously changing magnetic polarity, by passing an alternating current through a coil. If the alloy is brought into the electromagnetic alternating field of this magnet, magnetic induction gives rise to eddy currents in the alloy, and consequently to electromagnetic alternating fields.

When an alloy is introduced into the field of a magnet of constant polarity, eddy currents are likewise produced in it through electromagnetic induction if b/ the alloy is moved in the electromagnetic field of the magnet;

c/ the field of the magnet is moved about the alloy;

d/ both the magnetic field and the alloy are moved in relation to one another.

The fundamental measures described above for the production of electromagnetic alternating fields may also be varied very substantially, or combined with one another. This is illustrated by the following examples:

A. Solution heat treatment with the simultaneous action of two eddy currents of different frequencies on the alloys to be precipitation hardened The alloy to be precipitation hardened must, as a whole, be heated to the dissolution temperature. For optimum compliance with this requirement, one is not free any more to select the frequency of the induction current. The depth of penetration of the eddy current into the electrically conductive material is determined by the earlier given formula. From the formula it can be readily seen that, if all other values are constant, the depth of penetration is dependent only on the resonant frequency of the alloy in question.

At the present time, there are readily available alternating-current generators which produce currents of sufficiently high frequencies. Such generators are, for practical purposes, not limited in respect of their outputs, and are excellently suited for carrying out the described process. These are static converters which, with the aid of thyristors, convert an alternating current of the supply mains frequency (generally 50 or 60 c/s) into an alternating current of much higher frequency. Two basic types of such converters are manufactured, namely load-controlled converters with a sliding frequency and self-controlled converters having a fixed frequency.

Load-controlled converters are controlled in such a manner that they are automatically adjusted to the resonant frequency of the load, in this case to that of the alloy to be precipitation hardened. These converters work with a sliding frequency and, therefore, are excellently suited to the production of electromagnetic alternating fields which in turn give rise to specific oscillations of the atomic lattice with maximum efficiency.

The second type of converters works with a fixed but adjustable frequency. These converters serve to produce the required optimum heating frequency. The technical prerequisites for this type of solution heat treatment are thus fulfilled.

B. Solution heat treatment with the simultaneous action of a plurality of eddy currents of different frequencies on the alloys to be precipitation hardened It is known that the modulus of elasticity of an oscillatable structure which, in addition to other physical values, determines the resonant frequency does not have the same value in all directions. More complicated oscillation phenomena then occur than when a single modulus of elasticity is present. The oscillation pattern can be analyzed with an oscillograph. The oscillations resulting from the oscillogram can then be impressed on the alloys to be precipitation hardened by means of induction currents during the solution heat treatment.

An induction current of suitable frequency serves for heating, as exemplified earlier, and other induction currents, the frequencies of which are determined from the oscillograms, serve to produce particularly effective electromagnetic alternating fields.

In the cases referred to in the examples, the induction currents may either be simultaneously passed through a single induction coil or through a plurality of coils.

Instead of electromagnetic induction, it is also possible to apply other measures already indicated, or a combination thereof, for the purpose of producing electromagnetic alternating fields.

If eddy currents are generated in the alloys or alternating currents passed through them, these not only produce electromagnetic alternating fields but ensure that the heat required for heating to dissolution temperature is produced in the alloys themselves.

If the heat is produced in the alloys, the heating process and consequently the dissolution process take place substantially more quickly than when the heating is effected from outside, such as by heat conduction and heat radiation.

Another possible way of effecting additional heating,

particularly in the case of precipitation hardening with continuous movement of the alloys through the process, is to preheat the alloys and then to subject them to the action of electromagnetic alternating fields. However, this preheating may be effected only to temperatures at which there is no decisive ability to dissolve the alloying element required for the precipitation hardening.

. Up to these temperatures, even electromagnetic alternating fields have no substantial influence on the precipitation hardening process, since the prerequisite for precipitation hardening, i.e., decisive dissolution of the alloy element necessary for the precipitation hardening, is not fulfilled. The case of preheating occurs, for example, when ingots of an alloy pass through an extrusion press before the precipitation hardening, the pressing operation resulting in an increase in the alloy temperature. The use of inexpensive sources of energy for preheating purposes, for example natural gas, also results in a saving of more expensive power.

In principle, therefore, eddy currents, electric currents which are passed conductively through the alloys, heat conduction and heat radiation, as well as suitable combinations of these sources of heat, are available for carrying out the solution heat treatment required for the process according to the present invention.

The more quickly a dissolution process takes place, the higher the dissolution temperature must generally be, since the dissolution process is a diffusion process dependent on time and temperature. The two latter are interchangeable factors in a diffusion process and, since the speed of diffusion increases with rising temperature, increased temperature compensates for a shorter diffusion time in a rapidly conducted solution heat treatment, for example, in the case of inductive heating.

The uppermost limit for the permissible temperature and, consequently, the maximum speed at which the heating process may be conducted, are obtained in each individual case from the phase diagram of the alloy to be precipitation hardened, such as FIG. 3 reproduced herein for the exemplary CuAl alloy of which the pertinent data are described in connection with an exemplary manner 'of carrying out the inventive process. 1

It should be stressed that in the process according to the present invention, all the metallurgical processes required for precipitation hardening take place at the dissolution temperature. The alloys to be precipitation hardened must, therefore, be brough to that tempera ture. The most important parameter for optimum precipitation hardening is therefore the dissolution temperature.

Although not specifically expounded herein, it will be understood by those skilled in the art that the formula reproduced herein, forming the basis of the invention, allows all critical parameters to be calculated for specific alloys to be treated according to the invention. This of course includes the frequency of the electromagnetic alternating field (or fields) used in the process, the changing polarity of the magnet field (or fields) applied, the speed of the relative movement brought about between the field of the constantpolarity magnet (or magnets) and the alloys introduced into its (their) field, and/or the frequency of one of several simultaneously applied electric currents, which contributes to the heat applying step forming part of the inventive process. In the latter case the frequency of the remaining current (or currents) substantially corresponds to one (or more) discrete resonant frequency (frequencies) of the alloys under treatment.

The method according to the present invention provides a new technique for the precipitation hardening of Cu-Al alloys, while providing substantial technological and economic advantages, and can be applied to large-scale industrial production.

It should be understood, of course, that the foregoing disclosure relates only to preferred embodiments of the invention and that it is intended to cover all changes and modifications of the examples described, which do not constitute departures fromthe spirit and scope of the invention.

What I claim is:

1. A method of precipitation hardening of copperaluminum alloys, comprising the steps of subjecting the alloys to at least one electromagnetic alternating field, the frequency (f in c/s) of which is derived from the formula f= 503 /1! H- u,.

wherein d depth of penetration in millimeters, in proportion to the dimension of the alloys,

11.,- relative permeability H specific conductivity (Siemens) ohm", cm and simultaneously applying heat to the alloys in a single temperature zone lying just below the eutectoid temperature, and finally quenching the alloys to ambient temperature by an aqueous coolant, both said heat applying and quenching steps being in the order of seconds.

2. The precipitation hardening process as defined in claim 1, wherein said heat applying step is carried out for 5 to 60 seconds for alloys of a thickness of about 2 millimeters.

3. The precipitation hardening process as defined in claim 1, wherein said step of subjecting the alloys to the alternating field is carried out by electromagnetic induction.

4. The precipitation hardening process as defined in claim 1, wherein the frequency of the induction current is within a range of 500 to 2,000 c/s.

5. The precipitation hardening process as defined in claim 1, wherein said step of subjecting the alloys to the alternating field is carried out by conductively including the alloys in a circuit of a source of alternating current.

6. The precipitation hardening process as defined in claim 1, wherein said step of subjecting the alloys to the alternating field is carried out by introducing the alloys into the field of at least one magnet having a changing polarity, the frequency of which is derived from said formula.

7. The precipitation hardening process as defined in claim 1, wherein said step of subjecting the alloys to the alternating field is carried out by bringing about a relative movement between the field of at least one magnet having a constant polarity and the alloys introduced into said field, the relative speed of the movement being indirectly derived from said formula.

8. The precipitation hardening process as defined in claim 1, wherein said step of subjecting the alloys to at least one alternating field is effected by the simultaneous action of two electric currents, the frequency of 9. The precipitation hardening process as defined in claim 1, wherein said step of subjecting the alloys to at least one alternating field is effected by the simultaneous action of at least three electric currents, the frequency of at least two currents substantially corresponding to the discrete resonant frequencies of the alloys, while that of one other current is adapted to at least contributing to said heat applying step, this latter frequency being derived from said formula.

10. The precipitation hardening process as defined in claim 1, wherein said step of subjecting the alloys to the alternating field includes converting a mains-supply alternating current into a specific alternating current having a frequency in accordance with said formula, with static converter means having a sliding frequency output, including controlling the load of the converter means so as to regulate the frequency of the specific alternating current and adapt the same to the resonant frequency of the alloy load.

11. The precipitation hardening process as defined in claim 1, wherein said step of subjecting the alloys to the alternating field includes converting a mains-supply alternating current into a specific alternating current having a frequency in accordance with said formula, with self-controlled static converter means having an adjustable frequency output, including controlling the converter means so as to produce the required frequency for said heat applying step. 

2. The precipitation hardening process as defined in claim 1, wherein said heat applying step is carried out for 5 to 60 seconds for alloys of a thickness of about 2 millimeters.
 3. The precipitation hardening process as defined in claim 1, wherein said step of subjecting the alloys to the alternating field is carried out by electromagnetic induction.
 4. The precipitation hardening process as defined in claim 1, wherein the frequency of the induction current is within a range of 500 to 2,000 c/s.
 5. The precipitation hardening process as defined in claim 1, wherein said step of subjecting the alloys to the alternating field is carried out by conductively including the alloys in a circuit of a source of alternating current.
 6. The precipitation hardening process as defined in claim 1, wherein said step of subjecting the alloys to the alternating field is carried out by introducing the alloys into the field of at least one magnet having a changing polarity, the frequency of which is derived from said formula.
 7. The precipitation hardening process as defined in claim 1, wherein said step of subjecting the alloys to the alternating field is carried out by bringing about a relative movement between the field of at least one magnet having a constant polarity and the alloys introduced into said field, the relative speed of the movement being indirectly derived from said formula.
 8. The precipitation hardening process as defined in claim 1, wherein said step of subjecting the alloys to at least one alternating field is effected by the simultaneous action of two electric currents, the frequency of one current substantially corresponding to the resonant frequency of the alloys, while that of the other current is adapted to at least contributing to said heat applying step, this latter frequency being derived from said formula.
 9. The precipitation hardening process as defined in claim 1, wherein said step of subjecting the alloys to at least one alternating field is effected by the simultaneous action of at least three electric currents, the frequency of at least two currents substantially corresponding to the discrete resonant frequencies of the alloys, while that of one other current is adapted to at least contributing to said heat applying step, this latter frequency being derived from said formula.
 10. The precipitation hardening process as defined in claim 1, wherein said step of subjecting the alloys to the alternating field includes converting a mains-supply alternating current into a specific alternating current having a frequency in accordance with said formula, with static converter means having a sliding frequency output, including controlling the load of the converter means so as to regulate the frequency of the specific alternating current and adapt the same to the resonant frequency of the alloy load.
 11. The precipitation hardening process as defined in claim 1, wherein said step of subjecting the alloys to the alternating field includes converting a mains-supply alternating current into a specific alternating current having a frequency in accordance with said formula, with self-controlled static converter means having an adjustable frequency output, including controlling the converter means so as to produce the required frequency for said heat applying step. 